I own the 5D MK IV, and now the R. I've had it 2 weeks, and really like it for what I do. A lot of the way a person sees a camera is thru their filter for what they like to do.
While the camera is smaller, and has some new features that require learning, most of the points are non issues for me. I do miss the joystick, but I have already found with my 5D MK IV that the touch screen replaces it for a lot of uses. I also keep the M-Fn slider off because my hand is large and my thumb rests on it most of the time.
However, there are improvements too. All my lenses focus accurately from mfd to infinity, and focus is quick. NO MORE AFMA! I did not realize focus was quick, because I've had my old and slow to focus 50mm f/2.5 macro on the camera most of the time, it has a nice feel with that lens. This morning, the sun peeked out, so I decided to try the camera with my 100-400mm L MK II. I was shocked when it focused almost instantly. So, I tried focus from MFD to infiinity. That took less than a second, also as fast as my 5D MK IV.
With TC's. I decided to really stress autofocus, so I added my Canon 2X II TC on the 100-400 and set it to 400mm. Now, focus was slower from MFD to infinity took about 2-3 seconds, but it focused and did not hunt. Then, I tried a distance of about 20 ft to infinity, it was almost instant. Since that worked great, I added my 1.4X TC MK III stacked with the 2X. Then, I focused at mfd, the lens was still at infinity and took about 5 sec to focus at mfd. Slow but it did not hesitate or hunt. Then back to infinity and focus was the same, very slow but accurate and no hunting. I tried the 20 ft to infinity speed, it took less than a second. So, I turned on the 3 meter to infinity focus limiter to check focus speed from 3m to infinity, it was again very slow, maybe 4 seconds. Finally, I just tried focusing with things that were at least 50 ft away then switching to infinity. I was back into the 1 second or less speed again, and changing focus on different objects from 200 ft or 5000 ft to infinity, was, of course, instant.
So, The camera will focus reasonably well even at approximately f/16. The TC's slow down AF a lot, but thats the only case where autofocus is done with a small f-stop. To me, this means great AF at low light. Normally, even a 1.4X TC is something I avoid but its nice to see that I can use autofocus with both tc's stacked, since my eyesight is not nearly as accurate as autofocus with the R.
I have also tried AF with my 5D MK IV using similar combinations and live view. It will not autofocus with the stacked TC's unless I manually pre-focus to get it close, then it will snap into focus.